Impurity measuring method for Ge substrates

ABSTRACT

The present invention provides an impurity measuring method comprising the steps of dropping a drop of a first solution on the surface of a substrate to be measured, moving the drop dropped on the surface of the substrate so that the drop is kept in contact with the surface and collects an impurity absorbed on the surface, recovering the drop after the movement and analyzing the recovered drop by chemical analysis to determine the type and concentration of the impurity, characterized in that the first solution is phobic to the substrate and the substrate consists substantially of Ge. The method is of particular importance for measuring metallic contamination on the surface of Ge substrates.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/530,214, filed Dec. 12, 2003, the contents of which are herebyincorporated by reference in their entirety and are hereby made a partof this specification.

FIELD OF THE INVENTION

The present invention relates to a method for impurity measurements onGe substrates.

BACKGROUND OF THE INVENTION

In microelectronics industry Ge wafers are important substrates withtechnological applications in optical devices and are very recentlyintroduced as a replacement for Si substrates for advanced IntegratedCircuit (IC) devices. In order to realize high performance devices theGe wafer surface has to be of high purity.

Earlier studies have demonstrated the detrimental effects of metalliccontaminants on the electro-physical properties of the Ge surface.Therefore, it is needed to control carefully the metallic contaminationduring processing of Ge wafers. Specifications for metalliccontamination are set to levels below 5E9 at/cm2.

Cleaning recipes have to be developed and performances evaluated by anappropriate analysis methodology. Whereas several studies focused on theoptimization of cleaning recipes, there is still a lack in the metalliccontamination analysis for Ge wafers.

Direct-Total Reflection X-Ray Fluorescence (D-TXRF) is unrivalled forthe direct contamination analysis on Si wafers. The Detection Limits(DL) for this technique have been evaluated also for Ge wafers and arein the order of E10-E11 at/cm2. To meet the more stringent requirementsof the IC processing environment further development of the metrology ishence needed.

For the analysis of metallic contamination on Si wafers, the combinationof the pre-concentration method of Vapor Phase Decomposition—DropletCollection (VPD-DC) with micro-volume analytical techniques is awell-established method, which is patented by Maeda et al. (U.S. Pat.No. 4,990,459). The method consists of different subsequent steps.During the VPD step, the native oxide of the Si wafer is etched by an HFfume resulting in a hydrophobic Si surface. In the subsequent DC stepthe wafer surface is scanned with a micro-droplet of an aqueous mixtureto collect the metallic impurities. This droplet can then be analyzedwith any wet chemical micro trace analytical technique such as GraphiteFurnace—Atomic Absorption Spectrometry (GF-AAS) or Inductive CoupledPlasma—Mass Spectrometry (ICP-MS). Alternatively, the micro-droplet isdried on a carrier substrate and the resulted residue is analyzed byTXRF. The combination of VPD-DC and TXRF for Si wafers is studied indetail by C. Neumann and P. Eichinger (Spectrochim. Acta 46B, p1369,1991) and presented by D. Hellin et al. (‘Validation of VPD-DC formetallic contamination analysis of Si wafers’, TXRF2003 Conference,Hyogo, Japan). This technique, however, does not work in case of Gewafers, because an aqueous mixture is not phobic to Ge and droplet lossoccurs during scanning.

For metallic contaminants on small size Ge wafers (100 mm), apre-concentration method is based on the Droplet Sandwich Etch method(DSE), described by D. Hellin, et al. (‘Determination of metalliccontaminants on Ge wafers using Direct- and DSE-TXRF spectrometry’, Inpress, Spectrochim. Acta part B (2003)). In this method, a droplet of achemical mixture is deposited on a clean carrier substrate. Thesubstrate of interest is then placed on the carrier substrate with theside to be analyzed towards the carrier, sandwiching the droplet. Uponremoval of the top substrate a part of the chemical mixture remains onthe carrier substrate. This liquid can then be analyzed by anymicro-volume analytical technique. However, this methodology suffersfrom severe limitations with respect to automation and scalability tolarge wafer sizes.

The VPD-DC methodology as described by Maeda et al. requires the surfaceof the object to be measured to be hydrophobic to allow scanning by anaqueous solution. If the surface of the object is hydrophilic it has tobe rendered hydrophobic by a vapor phase treatment.

Since Ge substrates are not hydrophobic, both HCl and HF vaportreatments are tested to render the Ge surface hydrophobic.

This treatment resulted in rather hydrophilic surfaces. The contactangle after the treatments measured by dispensing a micro-droplet ofwater (in essence) onto the treated surface resulted in values of about15 deg. Consequently, the Ge surface could not be scanned by the waterdroplet as it wetted the surface and split into multiple smallerdroplets when scanning. In addition, the treated surface cannot bescanned with one of the solutions Maeda proposed: HF, HF+HNO₃, HF+H₂O₂and HCl+H₂O₂.

Solutions of HF up to 49 wt. % were tested.

After application of the VPD step, the Ge surface was not phobic to anyof these solutions and droplet loss occurred during scanning.

SUMMARY OF THE INVENTION

The present invention provides a method for measuring an impurity uponthe surface of a substrate comprising (or consisting of) the steps of:

-   -   dropping a drop of a first solution on said surface to be        measured,    -   moving the drop dropped on said surface so that the drop is kept        in contact with said surface and collects an impurity absorbed        on said surface,    -   recovering the drop after the movement, and    -   analyzing the recovered drop by chemical analysis to determine        the type and concentration of the impurity, characterized in        that the substrate consists essentially of Ge, and in that the        first solution is phobic to Ge substrate.

The method is of particular importance for measuring metalliccontamination on the surface of Ge substrates.

In a preferred method of the invention, said first solution, alsoreferred to as the “Ge-phobic solution”, comprises HCl and/or HBr.

Preferably, said Ge-phobic solution comprises HCl and/or HBr in aconcentration of at least about 30 wt. %, or at least about 35 wt. %.

A preferred Ge-phobic solution comprises at least about 37 wt. % HCl.

A preferred Ge-phobic solution comprises at least about 47 wt. % HBr.

Said Ge-phobic solution may further comprise less then about 0.25 wt. %H₂O₂, or may be free of germanium oxidizing compounds.

A method according to the invention may further comprise an additionalstep of making the surface of a Ge substrate phobic to the Ge-phobicsolution, thereby using a vapor of a second solution.

The second solution, also referred to as VPD-solution, may comprise ahalogen.

Said VPD-solution preferably comprises HF, HCl, or HBr or anycombination of two or three thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Contact angle as a function of the HCl concentration in themicro-droplet after HF VPD treatment of the Ge surface.

FIG. 2: Collection efficiency of VPD-DC applied on multi-element spincoated Ge wafers using a 37% HCl or 47% HBr DC solution.

FIG. 3(a): Influence of the HCl concentration and wafer temperature onthe removal of the Ge matrix and (b) related TXRF detection efficiencyof traces (K, Ca, Sc, Cr, Fe, Ni, Zn all at 2.5E12 atoms)

DETAILED DESCRIPTION

In a first aspect of the invention, an impurity measuring method for Gewafers is disclosed, comprising the steps of:

-   -   dropping a drop of a solution on the surface of a Ge substrate        to be measured, in which said solution is phobic to said Ge        substrate;    -   moving said drop dropped on the surface of said Ge substrate so        that said drop is kept in contact with the surface of said Ge        substrate and collects an impurity on the surface of said Ge        substrate;    -   recovering said drop after the movement; and    -   analyzing said recovered drop by chemical analysis to determine        the type and concentration of an impurity on the surface of said        Ge substrate.

Said method is of particular importance for measuring metalliccontamination on the surface of Ge substrates.

In a first embodiment, said Ge-phobic solution comprises HCl or HBr orany combination thereof.

In another embodiment said solution comprises at least 30 wt. % HCl orat least 30 wt. % HBr or any combination thereof.

In another embodiment the solution comprises HCl or HBr or anycombination thereof and less then 0.25 wt. % H₂O₂.

In a preferred embodiment, said Ge-phobic solution comprises 37 wt. %HCl.

In a second aspect of this invention, an impurity measuring method forGe wafers is disclosed, said method comprising the steps as disclosed inthe first aspect of this invention and an additional step making thesurface of a Ge substrate phobic to said impurity collecting solution,using a vapor of a second solution.

In a first embodiment of this second aspect, said Ge-phobic solutioncomprises HCl or HBr or any combination thereof.

In another embodiment of the second aspect, said solution comprises atleast 30 wt. % HCl or at least 30 wt. % HBr or any combination thereof.

In another embodiment of the second aspect, the solution comprises HClor HBr or any combination thereof and less then 0.25 wt. % H₂O₂.

In a preferred embodiment of this second aspect, said Ge-phobic solutioncomprises 37 wt. % HCl.

In another preferred embodiment of this second aspect, said secondsolution is of the form HX, in which X is F, Cl, or Br.

Thus, the present invention provides a method for measuring an impurityupon the surface of a substrate, comprising (or consisting of) the stepsof:

-   -   dropping a drop of a first solution on said surface to be        measured,    -   moving the drop dropped on said surface so that the drop is kept        in contact with said surface and collects an impurity absorbed        on said surface,    -   recovering the drop after the movement, and    -   analyzing the recovered drop by chemical analysis to determine        the type and concentration of the impurity, characterized in        that said substrate consists substantially of Ge and in that        said first solution is phobic to Ge substrates.

In the context of the present invention, the term “substrate essentiallyof Ge” refers to a Ge substrate, which may have on its surface nativegermanium oxides, and except in contexts where the native germaniumoxides are completely removed, the term “Ge substrate” may also refer toa substrate essentially of Ge.

In the context of the present invention, the term “phobic solution”refers to a solution that does not wet the Ge surface (or which is notabsorbed by the Ge substrate). Consequently, the droplet of saidsolution can be used to scan the Ge surface.

Preferably, in a method of the invention, said first solution, alsoreferred to as Ge-phobic solution, comprises HCl and/or HBr.

Preferably, said Ge-phobic solution comprises:

-   -   HCl in a concentration of at least about 20 wt. %, preferably of        at least about 25 wt. %, more preferably of at least about 30        wt. %, or at least about 35 wt. %,    -   HBr in a concentration of at least about 20 wt. %, preferably of        at least about 25 wt. %, more preferably of at least about 30        wt. %, or at least about 35 wt. %, or    -   HCl and HBr in (or to reach) a concentration of at least about        20 wt. %, preferably of at least about 25 wt. %, more preferably        of at least about 30 wt. %, or at least about 35 wt. %.

The contact angle can be increased significantly up to 55 degrees byaddition of HCl or HBr or any combination thereof to an aqueous phase.

FIG. 1 shows the contact angle values versus the concentration of HCl inthe micro-droplet after a HF VPD treatment. From about 20 wt. %, fromabout 25 wt. %, or from about 30 wt % HCl, the contact angle reachesvalues of at least 50 degrees.

This is also the case for HBr (not shown in FIG. 1).

A preferred Ge-phobic solution comprises at least about 37 wt. % HCl orat least about 47 wt. % HBr.

A Ge-phobic solution of the invention may further comprise less thenabout 0.25 wt. % H₂O₂, or may be free of germanium oxidizing compounds.

Ge oxidizing compounds in combination with the aqueous phase causesevere Ge etching. These Ge oxidizing compounds may be H₂O₂, HNO₃, O₂,03, or any compound forming germanium oxides.

In particular, the present invention provides a method for measuring animpurity upon the surface of a Ge substrate comprising (or consistingof) the steps of:

-   -   dropping a drop of a solution comprising HCl and/or HBr in a        concentration of at least 20 wt. %, preferably of at least 25        wt. %, more preferably of at least 30 wt. %, in particular of        (about) 37 wt. %, on said surface,    -   moving the drop dropped on said surface so that the drop is kept        in contact with said surface and collects an impurity absorbed        on said surface, recovering the drop after the movement,    -   and analyzing the recovered drop by chemical analysis to        determine the type and concentration of the impurity.

In a method of the invention, the movement of the drop may occur at aspeed of lower than about 20 mm/s, lower than about 10 mm/s, or about 5mm/s.

The analysis of the recovered drop may be performed by any wet chemicalmicro trace analytical technique such as Graphite Furnace—AtomicAbsorption Spectrometry (GF-AAS) or Inductive Coupled Plasma—MassSpectrometry (ICP-MS). Alternatively, the micro-droplet is dried on acarrier substrate and the resulted residue is analyzed by TXRF.

A method according to the invention may comprise, before the droppingstep, an additional step of treating the surface of a Ge substrate witha second solution in form of vapour, for etching, at least partially,the native germanium oxides.

Said second solution, also referred to as VPD-solution, preferablycomprise a halogen. In particular, said second solution may comprise HF,HCl or HBr or any combination thereof.

By etching, at least partially, the native germanium oxides, the surfaceof a Ge substrate is rendered more phobic to the phobic solution.

Removal of the Ge matrix increases the TXRF accuracy from a 20% level to100% values (FIG. 3).

When evaluating the collection efficiency of the method according to thepresent invention on metal spin coated Ge wafers (K, Ca, Cr, Fe, Ni andZn, 5E11-1E12 at/cm2 each), no metals above the detection limits of TXRFcan be measured. Thus, the values for collection efficiency ηColl aredetermined from the starting concentration CSpin and the detectionlimits of TXRF measurement tool CDL:$\eta_{Coll} \geq \frac{C_{Spin} - C_{DL}}{C_{Spin}}$

FIG. 2 shows collection efficiency values are very close to unity forall of the investigated elements.

Before chemical analysis of the recovered drop, the Ge matrix may beremoved at least partially. This may occur by chemical reaction with HCland/or HBr.

The VPD-DC step results in a micro-droplet containing a large Ge matrixconcentration of typically 10¹⁶-10¹⁷ atoms (i.e. 100-1000 mg/L). Thishigh matrix concentration is incompatible with the analysis ofultra-traces of contaminants within this matrix. Therefore the Ge matrixis removed at least partially, prior to the analysis.

In the case of VPD-DC-TXRF on Si wafers, the Si matrix is removed by aselective volatilization of SiF4 during the dry step.

Ge can however not be removed from aqueous solutions under speciation ofGeF4 but must be removed as the volatile GeCl4 or GeBr4. These compoundscan be formed in a reaction with HCl and/or HBr. Consequently, duringdroplet collection using HCl and/or HBr solutions, the Ge matrix reactsalready with HCl and/or HBr, resulting in the volatile product, whichthen is removed by heating the micro-droplet.

FIG. 3 indicates that drying of a Ge containing micro-droplet on ahotplate yields a high Ge matrix removal for HCl concentrations of 2 wt.% or higher. Starting with a Ge amount of 4×1016 atoms in the 50 μLdroplet, the residual Ge concentration in the residue is decreased withmore than three orders of magnitude. The method can be applied in alarge temperature window temperature of 60-90° C. Removal of the Gematrix increases the TXRF accuracy from a 20% level to 100% values.

A method according to the invention allows the evaluation of thecleanliness of Ge wafers in a practical and very sensitive manner andshould facilitate the development of cleaning recipes.

The invention is described in further details in the following example,which is intended for illustration purposes only, and should not beconstrued as limiting the scope of the invention in any way.

EXAMPLE

In this example, a 100 mm Ge wafers (Umicore) and 200 mm Si wafers, bothwith a <100> crystal orientation, were used.

Ge droplet residues standards were prepared by the micro-dropletdeposition method.

Standard multi-element solutions with a constant concentration of traces(K, Sc, Cr, Fe, Ni, Zn, all at 2.5×1012 atoms/50 μL) and a variable Geconcentration were prepared by dilution of ICP-MS calibration standards(1000 μg/mL, nitrate salts, Merck).

Droplets of 50 μL volume were pipetted on hydrophobic Si wafers (contactangle 65 deg) and dried at 50° C. and 100 mbar in a Wafer SurfacePreparation System (GeMeTec).

The accuracy of a TXRF measurement was evaluated as the ratio of themeasured amount of traces over the theoretical amount expected from thedilution of the standard solution.

The spin coating technique was used to prepare standard wafers.

Multi-element solutions were prepared by dilution of ICP-MS calibrationstandards (1000 mg/L, nitrate salts, Merck), acidified with HCl (2 vol.% of 37% HCl, Ultrex, Baker).

Portions of 10 mL were pipetted onto the Ge wafers positioned on ahigh-speed wafer spinner (Laurell WS-400). After 30 seconds, the spinnerwas run at 3000 rpm for 1 min in order to dry the wafer surface. Withthis procedure wafers with a target concentration in the order of1×11¹¹-1×10¹² at/cm² were prepared.

In the Ge matrix removal experiments, a Ge standard solution has beenprepared in ultra pure water. A Ge wafer piece was etched in a dilutedH₂O₂ solution (¼H₂O₂ (30%, Ashland)/H₂O) and the etched Ge amount wasdetermined from differential weighing. Trace amounts of K, Ca, Sc, Cr,Fe, Ni and Zn) were added from a dilution of ICP-MS standard solutions(1000 mg/L, nitrate salts, Merck).

TXRF analysis was performed using a FEI (formerly ATOMIKA) 8300 W systemequipped with a W or Mo tube operated at 50 kV and 55 mA.

All measurements were done in WLβ or MoKα excitation mode at 70% of thecritical angle, for 1000 s live time. Calibration was applied using a 1ng Ni micro-droplet type silicon wafer standard (FEI-ATOMIKA).

The VPD-DC preparation was done using an automated Wafer SurfacePreparation System (GeMeTec).

The DC step was performed using a 50 μL droplet of HCl (37%, Ashland,Megabit) which scanned the wafer surface twice.

Contact angles were determined by the sessile droplet method using a 5μL droplet of the tested solution. The contact angle was evaluated fromthe CCD camera picture using the Laplace-Young equation (Dataphysics,Contact Angle System OCA).

All references cited herein are incorporated herein by reference intheir entirety and are hereby made a part of this specification. To theextent publications and patents or patent applications incorporated byreference contradict the disclosure contained in the specification, thespecification is intended to supersede and/or take precedence over anysuch contradictory material.

The term “comprising” as used herein is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification and claims are to be understoodas being modified in all instances by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forthherein are approximations that may vary depending upon the desiredproperties sought to be obtained. At the very least, and not as anattempt to limit the application of the doctrine of equivalents to thescope of any claims in any application claiming priority to the presentapplication, each numerical parameter should be construed in light ofthe number of significant digits and ordinary rounding approaches.

The above description discloses several methods and materials of thepresent invention. This invention is susceptible to modifications in themethods and materials, as well as alterations in the fabrication methodsand equipment. Such modifications will become apparent to those skilledin the art from a consideration of this disclosure or practice of theinvention disclosed herein. Consequently, it is not intended that thisinvention be limited to the specific embodiments disclosed herein, butthat it cover all modifications and alternatives coming within the truescope and spirit of the invention.

1. A method for measuring an impurity upon a surface of a substrate, themethod comprising the steps of: dropping a drop of a first solution onthe surface to be measured; moving the drop dropped on the surface sothat the drop is kept in contact with the surface and collects animpurity absorbed on the surface; recovering the drop after themovement; and analyzing the recovered drop by a chemical analysis todetermine a type and a concentration of the impurity, wherein thesubstrate consists essentially of germanium, and wherein the firstsolution is phobic to the substrate.
 2. A method for measuring animpurity upon a surface of a substrate, the method comprising the stepsof: dropping a drop of a first solution on the surface to be measured,moving the drop dropped on the surface so that the drop is kept incontact with the surface and collects an impurity absorbed on thesurface, recovering the drop after the movement, and analyzing therecovered drop by a chemical analysis to determine a type and aconcentration of the impurity, wherein the substrate consistsessentially of germanium, and wherein the first solution is selectedfrom the group consisting of HCl, HBr, and a combination thereof.
 3. Themethod according to claim 2, wherein a concentration of the HBr, HCl, orcombination thereof in the first solution is at least about 30 wt. %. 4.The method according to claim 2, wherein a concentration of the HBr,HCl, or combination thereof in the first solution is at least about 35wt. %.
 5. The method according to claim 2, wherein the first solutioncomprises about 37 wt. % HCl.
 6. The method according to claim 2,wherein the first solution comprises about 47 wt. % HBr.
 7. The methodaccording to claim 2, wherein a concentration of the HBr, HCl, orcombination thereof in the first solution is at least about 30 wt. %,and wherein a concentration of H₂O₂ in the first solution is less thanabout 0.25 wt. %.
 8. The method according to claim 2, wherein aconcentration of the HBr, HCl, or combination thereof in the firstsolution is at least about 30 wt. %, and wherein the first solution isfree of germanium oxidizing compounds.
 9. The method according to claim2, wherein a germanium matrix is at least partially removed by achemical reaction with HCl, HBr, or mixtures thereof, before the step ofanalyzing is conducted.
 10. The method according to claim 2, wherein agermanium matrix is removed by a chemical reaction with HCl, HBr, orcombinations thereof.
 11. A method for measuring an impurity upon asurface of a substrate, the method comprising the steps of: making thesurface of the substrate phobic to a first solution by using a vapour ofa second solution; dropping a drop of the first solution on the surfaceto be measured; moving the drop dropped on the surface so that the dropis kept in contact with the surface and collects an impurity absorbed onthe surface; recovering the drop after the movement; and analyzing therecovered drop by a chemical analysis to determine a type aconcentration of the impurity, wherein the substrate consistsessentially of germanium, and wherein the first solution is phobic tothe substrate.
 12. A method for measuring an impurity upon a surface ofa substrate, the method comprising the steps of: making the surface ofthe substrate phobic to a first solution by using a vapour of a secondsolution; dropping a drop of the first solution on the surface to bemeasured; moving the drop dropped on the surface so that the drop iskept in contact with the surface and collects an impurity absorbed onthe surface; recovering the drop after the movement; and analyzing therecovered drop by a chemical analysis to determine a type and aconcentration of the impurity, wherein the substrate consistsessentially of germanium, and wherein the first solution comprises HCl,HBr, or combinations thereof.
 13. The method according to claim 12,wherein a concentration of the HBr, HCl, or combination thereof in thefirst solution is at least about 30 wt. %.
 14. The method according toclaim 12, wherein a concentration of the HBr, HCl, or combinationthereof in the first solution is at least about 35 wt. %.
 15. The methodaccording to claim 12, wherein the first solution comprises about 37 wt.% HCl.
 16. The method according to claim 12, wherein the first solutioncomprises about 47 wt. % HBr.
 17. The method according to claim 12,wherein a concentration of the HBr, HCl, or combination thereof in thefirst solution is at least about 30 wt. %, and wherein a concentrationof H₂O₂ in the first solution is less than about 0.25 wt. %.
 18. Themethod according to claim 12, wherein a concentration of the HBr, HCl,or combination thereof in the first solution is at least about 30 wt. %,and wherein the first solution is free of germanium oxidizing compounds.19. The method according to claim 12, wherein the second solutioncomprises a component selected from the group consisting of HF, HCl,HBr, and combinations thereof.
 20. The method according to claim 12,wherein a germanium matrix is at least partially removed by a chemicalreaction with HCl, HBr, or mixtures thereof, before the step ofanalyzing is conducted.
 21. The method according to claim 12, wherein agermanium matrix is removed by a chemical reaction with HCl, HBr, orcombinations thereof.